Seismotectonic study by relocation of the Wenchuan MS8.0 earthquake sequence
汶川MS8.0地震余震序列重新定位及其地震构造研究

Accurate relocation of the Wenchuan earthquake sequence is obtained by using the double difference relocation algorithm from the integrated travel time data recorded at the Western Sichuan movable seismic array and the earthquake emergency response stations. In this presentation, the seismotectonic deformation and rupture process is also investigated. The results show that the Wenchuan earthquake sequence begins from the south margin of the Pengguan Massif and extends northeastward for about 350 km within a width of 20~30 km. The spatial distribution of the aftershocks exhibits obvious inhomogeneity which can be divided into small sectors. The rigid Pengguan Massif controls the distribution of the earthquake sequence and the complex rupture process. The aftershocks occurred mainly in the depth ranging from 4 to 24 km inside the high velocity layer of the upper crust in the eastern border of the Longmen Shan and above the low velocity zone in the middle crust. Depth distributions of the aftershocks across the sections show clearly that the Yingxiu-Beichuan, Guanxian-Jiangyou and the Wenchuan-Maowen faults merge into a shear belt at the depth of 20~22 km. There exists an aftershock distribution belt from Xiaoyudong to the direction of Lixian, which is perpendicular to the strike of the Longmen Shan and longer than 60 km. Consulting with the results of the focal mechanism of strong aftershocks and the rupture process of the main shock, it could be inferred that the lower part of the Pengguan Massif has been broken up during the earthquake after long term stress accumulation, which would be the main reason why such a great earthquake occurred.